This invention relates generally to vehicle transmission gear shifter fork friction shoe inserts and more particularly to improved replaceable inserts providing increased insert life.
It is known to use replaceable plastic inserts fixed to gear shift fork bodies and adapted to engage an annular groove in a shiftable transmission element such as a synchronizer gear hub collar. An example of one such insert is found in U.S. Pat. No. 3,257,861 issued June 28, 1966 to M.E. Siefferman entitled Gear Shifter Fork assembly. The Siefferman patent discloses a single insert comprising a pair of axially spaced semicircular walls rigidly interconnected by a wall like bridge which loosely engages a notch in the fork to thereby prevent relative rotation between the insert and the fork. The notch is placed intermediate the circumferentially opposite ends of the fork flange whereby the insert and fork abutment surfaces are in substantially full circumferentially confronting or bearing contact to one another even though slight misalignment occurs between the fork and shifted element. The U.S. Pat. No. 4,238,012 issued Dec. 9, 1980 to Takeguchi et al. discloses a shift fork friction shoe insert that is tightly fixed to the main body and made of material having a high wear resistibility copper alloy such as high manganese brass. The insert is in the shape of a channel section and has elastomeric tongue-like projections extending from the bottom of the channel section. The projections are urged into grooves formed at the end parts of the fork main body.
The U.S. Pat. No. 4,495,831 issued Jan. 29, 1985 to Takashashi discloses a shift fork having an upwardly extending branch whose upper end is bevelled to introduce dripping lubricant to sliding surfaces of the collar and shift fork branch.
A conventional friction shoe insert is depicted in prior art FIGS. 1-5. FIG. 1 shows a portion of a transmission shift fork, generally indicated at "a", wherein each of its branch free ends "b" is provided with an insert "c" formed of suitable plastic material. The inserts "c" are channel shaped in horizontal section, as seen in FIG. 4, having a pair of side walls "d" joined by a base wall "e". FIG. 3 shows base wall inner face "f" formed with a pair of cross ribs "g" adapted for reception in a corresponding rectangular notch "h" provided in each free end "b".
It will be noted in FIG. 5 that the insert base wall inner face "f" is in flush contact with inboard surface "i" of the branch free end "b"while the insert cross ribs "g" are spaced from notch back wall "j". As seen in FIG. 5A the fork "a" is adapted to move a sleeve or collar shift element "k" on a transmission main shaft (not shown) by virtue of each fork insert "c" being received in an annular U-shaped groove "1" formed on the outer circumference of collar "k". Each side wall "d" of the insert defines a thrust face having a predetermined bearing load support area, i.e. the insert side wall area carrying the force of the sleeve and the fork. In FIG. 5A that the insert thrust face bearing load support area is shown as the confronting contact area of the annular groove "1" radial thrust surface portion in flush abutment with the opposed insert side wall portion "d" is indicated by radial dimension "n". FIG. 5A shows that a substantial portion of the axial thrust bearing area, denoted by dimension "n", comprises the plastic insert base wall "e" having a thickness indicated by dimension "0". It wil be appreciated that a substantial portion, in the order of forty percent in the present example, of the insert side wall abutment area is not backed or supported by the fork branch end "b" when subjected to axial thrust loads. As a result such prior art inserts "c" provide reduced service life and possible shear failure of the base wall "e" under extended shift induced thrust loading.